PFM Module – Circuit Surgery

Today, it is very easy to build our dream projects within the stipulated time, because almost all supporting components & circuits are readily available in pre-wired and compact modules. A short tour through ebay (or amazon) will bring the items at our doorstep within days. “DC to DC Converter Step Up 1.5 Volt to 5 Volt” is one such item I often purchased from online stores. With an input voltage of 0.9V to 5V DC, the module gives a stable 5V DC output through its USB socket. Using two AA batteries we can expect an output current of 500 to 600mA, and a single AA battery gives output current about 200mA. The conversion efficiency is upto 96%.

Last week, when inspired by the irresistible temptation to learn what is behind this wonderful module, I jumped into my circuit surgery lab (at midnight) with one module in my hand. Interested readers can follow this article to know more about my findings!

The module comprises of two capacitors (C1-C2),one resistor (R1), one inductor (L1), one rectifier diode (D2), one LED (D1), and an IC (U1). All of these components are in SMD form, except the USB ‘A’ female socket. Observed values of the components are:

    C1: 100nF (Input Filter)
    C2: 100uF/16V (Output Filter)
    L1: 470 (47uH Inductor)
    D2: SS14 (Schottky Diode)
    D1: Red LED (Input Power Indicator)
    R1: 102 (1K Resistor- LED Current Limitor)
    U1: E5 0P (smd marking code… what is the actual part number?)

Blueprint of the internal circuitry is shown here. Note that R1 in the circuit is the traditional current limiting resistor for D1, and the R1+D1 combination (not drawn in the blueprint) is connected across the input supply points, ie across capacitor C1.

Working of this minuscule DC-DC Boost converter module is based on PFM (pulse frequency modulation) technique. Pulse-Frequency Modulation (PFM) is a modulation method for representing an analog signal using only two levels (1 and 0). It is analogous to Pulse-Width Modulation (PWM), in which the magnitude of an analog signal is encoded in the duty cycle of a square wave. Unlike PWM, in which the width of square pulses is varied at constant frequency, PFM is accomplished using fixed-duration pulses and varying the repetition rate thereof.
A PFM converter is an alternative DC/DC power-converter architecture that uses a variable frequency clock to drive the power switches and transfer energy from the input to the output. Because the drive signal’s frequency is directly controlled to regulate the output voltage, this architecture is referred to as pulse-frequency modulation (PFM). DC/DC converters with constant-on-time or constant-off-time control are typical examples of the PFM architecture.

Since the IC (U1) is an smd chip labelled with an unusual marking code, it’s very hard to figure out (atleast for me) the actual part number of the device. Fortunately, an extra-long journey through internet helped a lot!

First PFM chip found with similar pin assignment of U1, is A7530, which is a CMOS-based PFM step-up DC-DC converter available in SOT-23, SOT-25, and SOT-89-3 package. Only three external components are necessary for this chip (an inductor, a Schottky diode and an output filter capacitor), and the output voltage can be adjusted from 2.5V ~ 6.0V (in 0.1V step). A 7530 in SOT-89-3 package is good for building the exact replica of the PFM module.

Here is the application circuit diagram of A7530 (A7530K3R-XXY series) from AiT Semiconductor Inc. Type & Value of CINis not very crucial, but a tantalum-capacitor is mandatory as COUT.

Second chip is S-8353/8354 series from Seiko Instruments Inc. The S-8353/8354 series is a CMOS step-up switching regulator which mainly consists of a reference voltage source, an oscillation circuit, power MOSFET, an error amplifier, a phase compensation circuit, a PWM control circuit (S-8353 Series) and a PWM / PFM switching control circuit (S-8354 Series). The S-8353/8354 series can configure the step-up switching regulator with an external coil, capacitor, and diode.

Today, almost all components are available in SMD version too, and even an average hobbyist can collect (and handle) them without too much difficulty. So, if you are in a plan to fabricate your own DC-DC Boost converter PFM module, I am sure, this article will expedite the construction process-from concept to completion